Instruments for detecting and counting particles are essential process control tools in semiconductor and semiconductor package manufacturing. Most such instruments employ a sampling technique where a small amount of fluid (e.g. a gas or a liquid) from a volume under test is drawn through an active region of a counter and the particles in the sample are counted. Provided flow rates are known and no particles are lost or gained from the sampling tube walls, these instruments yield reasonably accurate estimates of particle concentrations. In certain cases, however, it may not be possible to sample volumes of fluid from within an enclosure for particle concentrations. Examples of such volumes are vacuum chambers (where there is no fluid to sample) and furnaces where the insertion of sampling probes may be impractical. A sampling process may also produce misleading results where the total fluid volume is small (e.g., inside magnetic disk drives) so that the sample flow becomes large compared to the usual air exchange. This will result in erroneously low particle counts being indicated.
The prior art illustrates a number of optical systems for remote detection of microscopic particles. U.S. Pat. No. 4,737,652 to Faschingleitner et al. illustrates an optical system wherein reference and test beams are passed through a test volume, chopped and then detected to determine the presence of particles in the test beam. By passing the reference beam through a similar environment as the test beam, common mode noise and other perturbations of the test beam can be eliminated.
In U.S. Pat. No. 4,492,467 to Drain et al., a system for determining the size of spherical particles is described wherein back-scattered, circularly polarized light, reflected from a particle, is assessed. The system measures the angular intensity distribution of the back-scattered light and converts it to particle size.
U.S. Pat. No. 4,365,896 to Mihalow, a system is disclosed for correcting for optical attenuation that occurs in a beam used to interrogate an enclosed volume. Bacha1o, in U.S. Pat. No. 4,986,659, describes a method for measuring the size and velocity of microscopic particles, using the phase and intensity of scattered light. The Bachalo system is similar to a phase-Doppler velocimeter wherein a pair of coherent, identical frequency optical beams are caused to cross within a volume being examined, to create an interference pattern at the crossing point. By examining particle-scattered light from the region of interference, the presence of the particle is detected. Bachalo indicates that the scattered light is directed onto photo detectors that enable the phase of a Doppler burst to be detected. In addition, the amplitude of the Doppler burst is detected to enable a sizing of the particle.
A similar system to that described by Bachalo is shown in U.S. Pat. No. 4,373,807 to Gouesbet. However, one of the crossing beams has its frequency acousto-optically altered by a Bragg cell. As a result, the interference pattern created by the crossing beams exhibits fringe movement at the beat frequency between the beams, such that presence of a particle therein results in a signal offset from the beat frequency by the Doppler frequency. Both Bachalo and Gouesbet achieve their dual beams through the use of a beam splitter and require very precise alignment of their systems to achieve a prescribed beam crossing region.
Accordingly, it is an object of this invention to provide a submicron particle detector which eliminates the need for high precision optical system alignment.
It is another object of this invention to provide an improved optical particle detection system that is able to discriminate between weak signals from particles and much stronger ones from surface scatter and other stray light.
It is still another object of this invention to provide improved optical detection systems that can operate with forward and back-scattered optical energy.